Ceramic composite and methods of making the same

ABSTRACT

There is provided a method for producing a self-supporting ceramic composite comprising (1) a ceramic matrix obtained by oxidation of an aluminum zinc alloy to form a polycrystalline oxidation reaction product of the metal (10) with an oxidant, and (2) one or more fillers (12) embedded by the matrix. The metal alloy (10) and permeable mass of filler (12) having at least one defined surface boundary (14) are oriented relative to each other so that formation of the oxidation reaction product will occur into said mass of filler (12) and in a direction towards said defined surface boundary (14). On heating the metal (10) to a first temperature above its melting point but below the melting point of said oxidation reaction product to form a body of molten parent metal, the molten metal reacts with said oxidant to form said oxidation reaction product which infiltrates said mass of filler (12) to said defined surface boundary (14). The resulting infiltrated mass is heated to a second temperature in order to remove or oxidize at least a substantial portion of any residual non-oxidized metallic constituents from or in said infiltrated mass without substantial formation of said oxidation reaction product beyond said defined surface boundary (14), thereby producing a self-supporting ceramic composite.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention broadly relates to novel ceramic composites andmethods of making the same. In a more specific aspect, the inventionrelates to ceramic composites particularly useful as refractories, suchas steel plant refractories. The invention also relates to methods ofmaking the ceramic composites by the directed oxidation at elevatedtemperatures of a parent metal into a permeable mass of filler materialfollowed by a subsequent heating step to remove or oxidize residualnon-oxidized metal constituents.

2. Description of Commonly Owned Patent Applications and Background

The subject matter of this application is related to Marc S. Newkirk etal. U.S. Pat. No. 4,713,360 which issued on Dec. 15, 1987 and was basedon commonly owned U.S. patent application Ser. No. 818,943, filed Jan.15, 1986, which is a continuation-in-part of Ser. No. 776,964, filedSept. 17, 1985, which is a continuation-in-part of Ser. No. 705,787,filed Feb. 26, 1985, which is a continuation-in-part of Ser. No.591,392, filed Mar. 16, 1984, and entitled "Novel Ceramic Materials andMethods for Making the Same". This patent discloses the method ofproducing self-supporting ceramic bodies grown as the oxidation reactionproduct from a parent metal precursor. Molten parent metal is reactedwith a vapor-phase oxidant to form an oxidation reaction product, andthe metal migrates through the oxidation reaction product toward theoxidant thereby continuously developing a polycrystalline ceramic bodyof the oxidation reaction product. The ceramic body can be producedhaving metallic components and/or porosity, which may or may not beinterconnected. The process may be enhanced by the use of an alloyeddopant, such as in the case of an aluminum parent metal oxidized in air.This method was improved by the use of external dopants applied to thesurface of the precursor metal as disclosed in commonly owned andcopending U.S. patent applications Ser. No. 822,999, filed Jan. 27,1986, which is a continuation-in-part of Ser. No. 776,965, filed Sept.17, 1985, which is a continuation-in-part of Ser. No. 747,788, filedJune 25, 1985, which is a continuation-in-part of Ser. No. 632,636,filed July 20, 1984, all in the name of Marc S. Newkirk et al., andentitled "Methods of Making Self-Supporting Ceramic Materials".

The subject matter of this application is also related to that ofcommonly owned and copending U.S. patent applications Ser. No. 819,397,filed Jan. 17, 1986, which is a continuation-in-part of Ser. No.697,876, filed Feb. 4, 1985, both in the name of Marc S. Newkirk et al.and entitled "Composite Ceramic Articles and Methods of Making theSame". These applications disclose a novel method for producingself-supporting ceramic composites by growing an oxidation reactionproduct from a parent metal into a permeable mass of filler, therebyinfiltrating the filler with a ceramic matrix.

Further developments of the foregoing methods enable the formation ofceramic composite structures which (1) contain therein one or morecavities which inversely replicate the geometry of a shaped precursorparent metal, and (2) have a negative pattern which inversely replicatesthe positive pattern of a parent metal precursor. These methods aredescribed, respectively, (1) in Commonly Owned U.S. patent applicationSer. No. 823,542 filed Jan. 27, 1986, in the name of Marc S. Newkirk etal, entitled "Inverse Shape Replication Method of Making CeramicComposite Articles and Articles Obtained Thereby", and (2) in CommonlyOwned U.S. patent application Ser. No. 896,157, filed Aug. 13, 1986, inthe name of Marc S. Newkirk, and entitled "Method of Making CeramicComposite Articles with Shape Replicated Surfaces and Articles ObtainedThereby".

Also, methods of making ceramic composite structures having apre-selected shape or geometry were developed. These methods include theutilization of a shaped preform of permeable filler into which theceramic matrix is grown by oxidation of a parent metal precursor, asdescribed in Commonly Owned U.S. patent application Ser. No. 861,025,filed May 8, 1986, in the name of Marc S. Newkirk et al. and entitled"Shaped Ceramic Composites and Methods of Making the Same". Anothermethod of making such shaped ceramic composites includes the utilizationof barrier means to arrest or inhibit the growth of the oxidationreaction product at a selected boundary to define the shape or geometryof the ceramic composite structure. This technique is described inCommonly Owned U.S. patent application Ser. No. 861,024, filed May 8,1986, in the name of Marc S. Newkirk et al. and entitled "Method ofMaking Shaped Ceramic Composites with the Use of a Barrier".

The entire disclosures of all of the foregoing Commonly Owned PatentApplications and Patent are expressly incorporated herein by reference.

Common to each of these Commonly Owned Patent Applications and patent isthe disclosure of embodiments of a ceramic body comprising an oxidationreaction product, most typically interconnected in three dimensions,and, optionally, one or more non-oxidized constituents of the parentmetal or voids or both. The metal phase and/or the voids may or may notbe interconnected depending largely on such factors as the temperatureat which the oxidation reaction is allowed to proceed, the compositionof the parent metal, the presence of dopant materials, etc. For example,if the growth process is continued to substantially exhaust (convert)the metal constituents, porosity will result as a partial or nearlycomplete replacement of the metal phase throughout the bulk of thecomposite body, while developing a dense ceramic skin at the surface ofthe composite body. In such a case, the interconnected porosity istypically accessible from the surface of the ceramic body from whichmatrix development initiated.

Ceramic refractories are useful as components for applications requiringgood resistance to thermal shock, corrosion and erosion in contact withmolten metals. Such components may, for example, be used in controlmeans for regulating the flow of molten metals in molten metal transfersystems, for example, in the manufacture and handling of steel. Suchuses include, for example, slide gates, sub-entry nozzles, and ladleshrouds. Slide gates are used for controlling the flow of molten metalfrom a ladle. Generally, slide gate systems including some rotarydesigns, consist of a fixed nozzle attached to and within a movableplate. The flow of molten metal from a ladle is controlled by moving themovable plate to fully or partially align openings. When filling theladle and during shut-off, the openings are misaligned. The principaladvantage of the slide gate system over a conventional stopper rodsystem is its improved reliability of shutoff, ability to modulatemolten metal flow, and lack of aspiration of the molten steel productstream. However, even the best slide gate systems, such as high-aluminaslide gate systems, are inadequate for certain molten metals, such asspecialty steel like low-carbon, high-manganese grades. These corrosivesteel compositions will seriously attack the bonding media used in mosthigh-alumina grade slide gate systems.

Today, in the U.S. market, the majority of the slide gate refractoriesare composed of either tar-impregnated high-alumina, or fired magnesiamaterials. However, such slide gate refractories do not possess thethermal shock, corrosion and erosion resistance criteria to stand up tolong ladle holding and teeming times and preheating, and therefore havea short service life.

The ceramic composites of this invention offer potential for use assteel plant refractories such as slide gate refractories, that do nothave the foregoing deficiencies while still possessing thermal shock,corrosion and erosion resistance criteria to withstand long ladleholding and teeming times and preheating. In addition, they may beuseful for other applications requiring thermal shock resistance andhigh temperature strength retention.

SUMMARY OF THE INVENTION

In accordance with the present invention, there is provided a method forproducing a self-supporting ceramic composite comprising (1) a ceramicmatrix obtained by oxidation of a parent metal comprising analuminum-zinc alloy to form a polycrystalline material consistingessentially of an oxidation reaction product of the parent metal with anoxidant, and (2) a filler embedded by the matrix.

Generally, a precursor metal and permeable mass of filler are orientedrelative to each other so that the growth of a polycrystalline materialresulting from the oxidation of a precursor metal (hereinafter referredto as the "parent metal" and defined below) as described in theabove-referenced Commonly Owned Patent Applications is directed towardsand into a permeable mass of filler material. (The terms "filler" and"filler material" are used herein interchangeably.) The mass of fillerhas at least one defined surface boundary and is infiltrated withpolycrystalline material to the defined surface boundary to provide aceramic composite. Under the process conditions of this invention, themolten parent metal oxidizes outwardly from its initial surface (i.e.,the surface exposed to the oxidant) towards the oxidant and into themass of filler by migrating through its own oxidation reaction product.The oxidation reaction product grows into the permeable mass of filler.This results in novel ceramic matrix composites comprising a matrix of aceramic polycrystalline material embedding the filler materials.

The parent metal used in the ceramic matrix growth process comprises analuminum alloy having at least about 1% by weight zinc, and this parentmetal is heated to a first temperature above its melting point but belowthe melting point of the oxidation reaction product thereby forming abody or pool of molten parent metal which is reacted with an oxidant,preferably a vapor-phase oxidant, e.g., air, to form the oxidationreaction product. At this first temperature or within this firsttemperature range, the body of molten metal is in contact with at leasta portion of the oxidation reaction product which extends between thebody of molten metal and the oxidant. Molten metal is drawn through theoxidation reaction product towards the oxidant and towards and into themass of filler material to sustain the continued formation of oxidationreaction product at the interface between the oxidant and previouslyformed oxidation reaction product. The reaction is continued for a timesufficient to infiltrate the filler material to the defined surfaceboundary with the oxidation reaction product by growth of the latter,which has therein inclusions of non-oxidized metallic constituents ofthe parent metal.

The resulting ceramic composite comprises a filler and a ceramic matrixwhich is a polycrystalline oxidation reaction product and containsresidual non-oxidized constituents of the parent metal, most typicallyaluminum and zinc but also may include other metals as well. The ceramiccomposite is heated to a second temperature (or within this secondtemperature range) above the first temperature, but below the meltingpoint of the oxidation reaction product, in order to remove or oxidizeat least a substantial portion of the residual non-oxidized metallicconstituents, as by volatilization or oxidation of the metalconstituents, from the polycrystalline material without substantialformation of the oxidation reaction product beyond the defined surfaceboundary. Heating to this second temperature may be carried out eitherin vacuum, an inert atmosphere, or more preferably, an oxygen-containingatmosphere or, most preferably, air. Some of the removed metal phase isreplaced essentially by porosity or voids. Other metal phases areoxidized in situ, converting the metal to an oxidized species. The finalstructure comprises a ceramic matrix and filler material, and theceramic matrix consists essentially of oxidation reaction product andinterconnected porosity with at least a portion being accessible fromone or more surfaces of the ceramic composite. Preferably, the surfaceporosity is characterized by openings having a mean diameter of lessthan about 6 microns, which prevents the penetration of some materialssuch as molten steel.

The products of the present invention are essentially ceramic; that is,essentially inorganic and substantially void of metal, although theremay be some inclusions or islands of metal. The products are adaptableor fabricated for use as refractories, which, as used herein, areintended to include, without limitation, industrial slide gate valverefractories that slidably contact the bottom portion of a vessel, ladleor the like, containing molten metal, such as steel, to permit andregulate the flow of molten metal through an aperture in the ladle.

As used in this specification and the appended claims, "oxidationreaction product" means the product of reaction of metals with anoxidant thereby forming an oxide compound.

As used herein and in the claims, "oxidant" means one or more suitableelectron acceptors or electron sharers and may be a solid, a liquid or agas (vapor), or some combination of these at the process conditions.

The term "parent metal" as used in this specification and the appendedclaims refers to that aluminum alloy metal having typically at leastabout 1 to 10 percent by weight zinc and which is the precursor of thepolycrystalline oxidation reaction product and includes that aluminumalloy metal, and commercially available aluminum alloy metal havingtypically at least about 1 to 10 percent by weight zinc, as well asimpurities and/or alloying constituents therein.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic, cross-sectional view in elevation showing anassembly of an aluminum alloy parent metal, overlaying filler materialand a support bed contained in a refractory crucible; and

FIG. 2 is a partial schematic, vertical cross-sectional view showing aslide gate valve, slidably disposed between a top plate of the bottomportion of a ladle and a tube holder that holds a tube through whichmolten metal passes after leaving the ladle.

DETAILED DESCRIPTION OF THE INVENTION

Referring to the drawings for the practice of the present invention, inFIG. 1 a parent metal 10, comprising an aluminum alloy having at leastabout 1 to about 10 percent by weight zinc, is formed into an ingot,billet, rod, plate or the like. This body of parent metal 10 and apermeable mass of filler material 12 having at least one defined surfaceboundary 14 are positioned adjacent to each other and oriented withrespect to each other so that growth of the oxidation reaction productwill be into the filler material 12 and in a direction towards thedefined surface boundary 14 in order that the filler material 12, or apart thereof, will be infiltrated by the growing oxidation reactionproduct. The parent metal 10 and filler material 12 are embedded in asuitable support material 16 substantially inert under the processconditions and of such constituency so that oxidation reaction will notproceed into this bedding, and the upper or exposed surface of the massof filler is flush with the surface of the bedding. Suitable beddingmaterials include, for example, certain grades of particulate aluminasuch as 38 Alundum manufactured by Norton Company. The assembly orlay-up is contained in a suitable refractory vessel or crucible 18.

The filler material 12 preferably comprises ceramic or refractorymaterial and may be a lattice or array of a bed of particulates,granules, powders, aggregate, refractory fiber cloth, fibers, tubes,tubules, pellets, whiskers, or the like, or a combination of theforegoing. The array or arrangement of filler material(s) 12 may beeither loose or bonded and has interstices, openings, interveningspaces, or the like, to render it permeable to the oxidant and to theoxidation reaction product growth. Further, suitable filler(s) dependingupon specific end use of the product, may include for example, metaloxides, borides, nitrides, or carbides of a metal selected from thegroup consisting of aluminum, cerium, hafnium, lanthanum, silicon,neodymium, praseodymium, samarium, scandium, thorium, uranium, titanium,yttrium, and zirconium. Certain of these fillers may require protectivecoatings to prevent their reaction and/or oxidation under the conditionsof the process. In one embodiment of the invention, the filler includesfrom about 3 percent to 10 percent by weight of silica, such as incombination with alumina. Alumina filler found especially useful has amesh size of from about 5 to 500 (U.S. standard sieve). Silicon carbideas filler may have a mesh size of from about 500 to about 1000 (U.S.standard sieve).

The assembly is, in any case, arranged so that growth of the oxidationreaction product will occur into the filler material 12 such that voidspace between filler particles will be substantially filled by the grownoxidation reaction product. A matrix of the polycrystalline materialresulting from the oxidation reaction product growth is simply growninto and/or around the filler material 12 so as to embed and infiltratethe latter preferably to its defined surface boundary 14 withoutsubstantially disturbing or displacing the filler material 12. Thus, noexternal forces are involved which might damage or disturb thearrangement of the filler material 12 and no awkward and costly hightemperature, high pressure processes and facilities are required as inknown conventional processes to achieve a dense composite ceramicstructure. In addition, the stringent requirements of chemical andphysical compatibility necessary for pressureless sintering to formceramic composites are greatly reduced or eliminated by the presentinvention.

A solid, liquid, or vapor-phase oxidant, or a combination of suchoxidants may be employed. Vapor-phase oxidants include, withoutlimitation, oxygen, oxygen-argon, or other inert gas mixtures and air.

Solid oxidants include reducible oxides such as silica, tin oxide, orzinc oxide. When a solid oxidant is employed, it is usually dispersedthrough the entire bed of filler or through a portion of the bedadjacent to the parent metal, in the form of particulates admixed withthe filler, or perhaps as coatings on the filler particles.

If a liquid oxidant is employed, the entire bed of filler or a portionthereof adjacent to the molten metal is coated or soaked as by immersionin the oxidant to impregnate the filler. A suitable liquid oxidantincludes low melting glasses.

Zinc as a dopant material (which is described below in greater detail)promotes or facilitates growth of the oxidation reaction product andsubsequent removal of the non-oxidized metallic constituents from theoxidation reaction product initially formed. The zinc dopant is alloyedinto the aluminum parent metal, and comprises from about 1 percent byweight to about 10 percent by weight, and preferably about 4 percent toabout 7 percent by weight. Additional dopant materials (as disclosed inthe aforementioned. Commonly Owned Patent Application and Patent may beused in conjunction with the parent metal 10 as by alloying dopantmaterial with the parent metal 10, applying an external coating to thesurface of the parent metal 10, or by incorporating or mixing the dopantmaterials with the filler material(s) 12. For example, magnesium may beused to augment the dopant action of zinc.

Referring to FIG. 1, a body of aluminum parent metal 10 along with themass of permeable filler material 12 are positioned in a crucible orother refractory container 18 such that at least one metal surface ofthe parent metal 10 is exposed to the adjacent to or surrounding mass offiller material 12. If a vapor-phase oxidant is used, the mass of filleris permeable to the gaseous oxidant present in the oxidizing atmosphere(typically air at ambient atmospheric pressure). The resulting assemblyis then heated to a first temperature range in the presence of theoxidant in a suitable furnace (not shown in the drawings) to elevate thetemperature thereof in the region, typically, with air as the oxidant,between about 850° C. to about 1450°C., or more preferably, betweenabout 950° C. to about 1100° C. to form a pool or body of molten parentmetal. The temperature region depends upon the filler material 12,dopant or dopant concentrations, oxidant, or the combination of any ofthese. At this temperature region parent metal transport begins to occurthrough the oxide skin normally protecting the aluminum parent metal.

The continued high temperature exposure of the parent metal 10 to theoxidant allows the continued oxidation of parent metal 10 to form apolycrystalline oxidation reaction product of increasing thickness. Thisgrowing oxidation reaction product progressively infiltrates thepermeable mass of filler material 12 with an interconnected oxidationreaction product matrix which also may contain non-oxidized parent metalconstituents, thus forming a cohesive composite. The growingpolycrystalline matrix impregnates or infiltrates the filler material 12at a substantially constant rate (that is, a substantially constant rateof thickness increase over time), provided there is a relativelyconstant source of oxidant, for example, by allowing a sufficientinterchange of air (or oxidizing atmosphere) in the furnace. Interchangeof oxidizing atmosphere, in the case of air, can be convenientlyprovided by vents in the furnace. Growth of the matrix continues for atime sufficient for the polycrystalline oxidation reaction product toinfiltrate the mass of filler material 12 to the defined boundary 14,which preferably occurs when substantially all of the parent metal 10 isconsumed, i.e., substantially all of the parent metal 10 has beenconverted into the matrix.

The ceramic composites initially produced by the oxidation of thealuminum alloy parent metal with the oxidant comprises the fillermaterial(s) infiltrated and embedded, preferably to the definedboundary, with the polycrystalline oxidation reaction product of theparent metal with the oxidant, and one or more non-oxidized metallicconstituents of the parent metal including aluminum and zinc, and othermetals depending on the parent metal composition. The volume percent ofresidual metal (non-oxidized metallic constituents) can vary over a widerange depending on whether or not the oxidation reaction process isconducted largely to exhaust aluminum alloy parent metal. By way ofexample only, a ceramic composite formed from aluminum alloy metal and50 volume percent filler processed in air at about 1000° C. may containabout 0.5 to 10 volume percent residual metal.

In order to produce a ceramic composite substantially devoid of metallicconstituents, such as for a composite used for slide gate valverefractories, the non-oxidized metallic constituents (residual metal)present after the first heat treatment are substantially removed and/oroxidized in situ by a second or subsequent heating step. The initiallyformed ceramic composite is heated at a temperature higher than thetemperature first employed in forming the initial ceramic composite.This second heating step may be accomplished by elevating thetemperature to effect the substantial volatilization and/or oxidation ofthe residual metal. This second heating step may be carried out in anoxygen-containing or inert atmosphere or in a vacuum. Anoxygen-containing atmosphere is preferred because removal of residualmetal by oxidation thereof can be effected at a lower temperature thanremoval by volatilization in an inert atmosphere or in a vacuum. Air atambient atmospheric pressure is most preferred for reasons of economy.

The assembly is heated in the furnace in the presence of the desiredatmosphere to elevate the temperature thereof in the region typicallybetween about 1250° C. to about 2000°C.; more preferably at least about1400°C., or from about 1400° C. to about 1600°C. This temperature ishigher or above the temperature that was employed to produce theinitially formed ceramic composite. At these elevated temperatures, anyresidual non-oxidized metallic constituents of the aluminum alloy parentmetal are essentially removed or converted to an oxide without anyfurther growth beyond the defined surface boundary. It is believed thata majority of the residual non-oxidized metallic constituents areessentially helped to be removed through volatilization of the zincdopant. Some of the residual aluminum metal will oxidize in situ withouteffecting the defined boundary of the part. The zinc dopant not onlypromotes or facilitates growth of the oxidation reaction product, butvolatilizes at elevated temperature, generating porosity and highsurface area which then promotes oxidation of residual non-oxidizedmetallic constituents of the aluminum alloy parent metal leaving minimalresidual metal in the composite.

As was previously mentioned, the amount of zinc that is to be alloyedinto the aluminum parent metal preferably comprises from about 4 percentby weight to about 7% by weight (based on the weight of the aluminumparent metal labelled as 10). The zinc may be alloyed directly withunalloyed commercial purity aluminum, e.g., of 99%, 99.5% or 99.7%grade. If so desired, high or super purity aluminum, e.g., 99.9% orpurer, may be used as a base for the alloying addition. This may bedesirable where the refractory end-product is to be used in conjunctionwith very high purity molten metals where even traces of contaminantsare unwanted. On the other hand, certain zinc-containing commercialwrought alloys, e.g., of the Aluminum Association 7000 series or castingalloys, e.g., of the Aluminum Association 700 series may be used wherethe zinc content is above 1.0%, preferably above 4.0%, and where thepresence of other alloying elements is not harmful to the end use. Forexample, alloy 7021 which contains 5.0-6.0% zinc, 1.2-1.8% magnesium,0.08-0.18 % zirconium with permitted maxima for the following elements:silicon 0.25%; iron 0.40%; copper 0.25%; manganese 0.10%; chromium0.05%; titanium 0.10%; other elements each 0.05%; up to a total of 0.15%(all percentages by weight) the balance being aluminum, is one amongseveral such alloys which would comprise a suitable parent metal for theinvention. In this case, the magnesium present in the alloy augments thedopant action of zinc.

When desired, the composite may be cooled and removed from the furnace.The cooled body may then be machined. (e.g., such as by milling,polishing, grinding or the like) on one or more surfaces to desiredtolerances. This alternative may be particularly desirable in themanufacture of ceramic articles requiring close tolerances.

In one preferred embodiment of the present invention, displayed in FIG.2 the ceramic composites of the invention can be fabricated for use asslide gate valve refractories. The slide gate valve, generallyillustrated as 20 in FIG. 2, contacts a top plate 22 or the bottomportion of a ladle, generally illustrated as 24, containing molten metal26 (i.e., molten steel). Top plate 22 is integrally bound to the ladle24 and has a top plate aperture 28 which is in direct communication witha ladle aperture 30 disposed in the bottom of the ladle 24. The slidegate valve 20 has a slide gate structure 32 with at least one slide gateaperture 34. A drive means 36, such as a throttling cylinder, or thelike, is coupled to the slide gate 20 to slide (or rotate) the slidegate along the bottom surface of the top plate 22 to either align ormisalign the slide gate aperture 34 with the top plate aperture 28 andthe ladle aperture 30. A tube holder means, generally illustrated as 40,holds a tube 38 and supports the slide gate valve 20, the top plate 22,and the ladle 24 that is bound to the top plate 22. Tube 38 conducts theflow of molten metal 26 after the same leaves ladle 24 through slidegate 20. If the slide gate valve refractory 20 is disposed by the drivemeans 36 such that the aperture 34 of the slide gate valve refractory 20is totally misaligned with top plate aperture 28 and with ladle aperture30 of the ladle 24, molten metal 26 will not flow from the ladle 24.Also, molten metal 26 (as will be explained in greater detailhereinafter) will not penetrate into and through the porosity of theceramic matrix in the structure 32 of the slide gate valve 20. Asdepicted in FIG. 2 by the label 34 which is connected to a dotted line,when the slide gate valve 20 is slidably positioned along the top plate22 and the bottom portion of the ladle 24 such that the slide gateaperture 34 is generally aligned with the top plate aperture 28 and withladle aperture 30 of the ladle 24, molten metal 26 will flow by gravityfrom the ladle 24 through the respective apertures into the tube 38.

The slide gate structure 32 must be extremely flat, i.e., to withintolerances of 1/2000 of an inch or less, and must be held tightlyagainst the bottom surface of the top plate 22 so that molten metal willnot leak out between the contacting surfaces. The slide gate structure32, as well as the structure of the top plate 22, is composed ofrefractory materials or components that are capable of being machined(such as by milling, grinding, polishing, or the like) extremely smoothso the structure of the top plate 22 and the structure 32 of the slidegate valve 20 cannot pull out the grains of the other during opening andclosing of the slide gate valve 20 with the coupled drive means 36. Thestructure 32 of the slide gate valve 20 should not have pores which aretoo large since molten metal would penetrate the pores and weaken thestructure 32. Furthermore, the slide gate structure 32 must possessextremely good thermal shock resistance and must be composed ofrefractory materials or components that are strong enough to resistchemical corrosion and erosive effects from flowing molten metalcompositions. In order to fabricate a slide gate structure 32 from aceramic composite possessing the foregoing properties and/or criteria,the ceramic composite should contain a ceramic matrix substantiallyconsisting essentially of non-metallic and inorganic material(s). Anysubstantial amount of non-oxidized metallic constituents within aceramic composite, such as aluminum, could be detrimental to theperformance of the material by lowering its high temperature strength,possibly exhibiting oxidation overgrowth beyond the slide gatedimensions and causing the gate components to bond together, as well asaffecting thermal shock performance. Hence, the slide gate valve 20would fail in its function or have to be replaced after minimal use,most likely due to spalling, cracking, or surface overgrowth.

The ceramic composite structure obtained after removing and/or oxidizingsubstantially all of the residual non-oxidized metallic constituents ofthe aluminum parent metal is a coherent ceramic composite typicallyhaving from about 5% to about 98% by volume of the total volume of thecomposite structure comprised of one or more of the filler materialembedded within a polycrystalline ceramic matrix. The polycrystallineceramic matrix is comprised of about 94.5% or more by weight (of theweight of polycrystalline oxidation reaction product) of interconnectedalpha-alumina, about 5% or less of zinc aluminate, and about 0.5% orless by weight of non-oxidized metallic constituents of the aluminumparent metal.

The polycrystalline ceramic matrix exhibits some porosity ranging fromabout 2% by volume to about 25% by volume of polycrystalline ceramicmatrix, preferably not more than about 10%. It is believed that someporosity is required in order to provide the desired thermal shockresistance of the refractory product. At least a portion of the porosityis accessible from the surface, and typically about 5% of such porosityhave pore openings whose diameter measures from about 1 micron to about8 microns. Preferably, the openings of the porosity accessible from thesurface have a mean diameter of about 6 microns or less, where 6 is themean of a normal Gaussian distribution curve. An alumina-based ceramiccomposite having openings on its surface that measure about 6 microns orless in diameter is particularly useful in fabricating a slide gaterefractory since molten steel will not penetrate its structure.

The ceramic composite structure of this invention possesses thefollowing properties: a three-point bend test for hot Modulus of Rupture(MOR) of from about 3500 psi to about 6500 psi at 2550° F. (1400° C.) inN₂, depending on the size of the alumina filler material; a thermalshock resistance parameter (resistance to crack propagation, Rst) ofabout 60° F./in. 1/2; a volume stability (thermal expansion inaccordance with ASTM E228.71 from room temperature to 1500° C. and thencooled) of 0.15% or less in linear change with no rate changes thatresult in cracking or distortion; and a corrosion resistance (air/metalline wear in inches with a major diagonal 1×1 inch bar, 20 min. spintest, Al-killed steel, as described in the example below) of 0.04 inchor less.

The ceramic composite of this invention exhibits substantially cleangrain boundaries wherein the grain boundaries at the interconnection ofthe crystallites have no other phase present. Most notably, the grainboundaries are devoid of any siliceous phase. This feature isparticularly important for steel plant refractories. Low-meltingsilicates are found in almost every traditional alumina refractory, andthis material reacts with the molten iron causing dissolution into theliquid steel and ultimately leading to cracking, spalling and failure ofthe structure.

In addition, the composites of the present invention do not requireextra precautions to prevent oxidation of the bonding phase because itis a fully oxidized matrix, which is in contrast to carbon-bondedalumina refractories presently being used in Japan in the slide gatemarket.

A particularly effective method for practicing this invention involvesforming the filler into a preform with a shape corresponding to thedesired geometry of the final composite product. The preform may beprepared by any of a wide range of conventional ceramic body formationmethods (such as uniaxial pressing, isostatic pressing, slip casting,sedimentation casting, tape casting, injection molding, filament windingfor fibrous materials, etc.) depending largely on the characteristics ofthe filler. Initial binding of the particles prior to infiltration maybe obtained through light sintering or by use of various organic orinorganic binder materials which do not interfere with the process orcontribute undesirable by-products to the finished material. The preformis manufactured to have sufficient shape integrity and green strength,and should be permeable to the transport of oxidation reaction product,preferably having a porosity of between about 5 and 90% by volume andmore preferably between about 25 and 50% by volume. Also, an admixtureof filler materials and mesh sizes may be used. The preform is thencontacted with molten parent metal on one or more of its surfaces for atime sufficient to complete growth and infiltration of the preform toits surface boundaries.

As disclosed in copending U.S. patent application Ser. No. 861,024,filed on May 8, 1986, in the names of Marc S. Newkirk et al and entitled"Method of Making Shaped Ceramic Composites with the Use of a Barrier"and assigned to the same owner, a barrier means may be used inconjunction with the filler material or preform to inhibit growth ordevelopment of the oxidation reaction product beyond the barrier. Afterthe first heat treating step and before the second heating step, thebarrier is removed by any suitable means. Suitable barriers may be anymaterial, compound, element, composition, or the like, which, under theprocess condition of this invention, maintains some integrity, is notvolatile, and preferably is permeable to the vapor-phase oxidant whilebeing capable of locally inhibiting, poisoning, stopping, interferingwith, preventing, or the like, continued growth of oxidation reactionproduct. Suitable barriers for use with aluminum parent metal includecalcium sulfate (plaster of paris), calcium silicate, and Portlandcement, and mixtures thereof, which typically are applied as a slurry orpaste to the surface of the filler material. A preferred barriercomprises a 50/50 admixture of plaster of paris and calcium silicate.These barrier means also may include a suitable combustible or volatilematerial that is eliminated on heating, or a material which decomposeson heating, in order to increase the porosity and permeability of thebarrier means. The barrier is readily removed from the composite as bygrit blasting, grinding, etc.

As a result of using a preform, especially in combination with a barriermeans, a net shape is achieved, thus minimizing or eliminating expensivefinal machining or grinding operations.

As a further embodiment of the invention and as explained in theCommonly Owned Patent Applications, and patent the addition of dopantmaterials in conjunction with the parent metal can favorably influencethe oxidation reaction process. The function or functions of the dopantmaterial can depend upon a number of factors other than the dopantmaterial itself. These factors include, for example, the particularparent metal, the end product desired, the particular combination ofdopants when two or more dopants are used, the use of an externallyapplied dopant in combination with an alloyed dopant, the concentrationof the dopant, the oxidizing environment, and the process conditions.The dopant(s) used in the process should be substantially removed oroxidized during the second heating step so as to not adversely affectthe properties of the end product.

The dopant or dopants used in conjunction with the parent metal (1) maybe provided as alloying constituents of the parent metal, (2) may beapplied to at least a portion of the surface of the parent metal, or (3)may be applied to the filler bed or preform or to a part thereof, or anycombination of two or more of techniques (1), (2) and (3) may beemployed. For example, an alloyed dopant may be used in combination withan externally applied dopant. In the case of technique (3), where adopant or dopants are applied to the filler bed or preform, theapplication may be accomplished in any suitable manner, such as bydispersing the dopants throughout part or the entire mass of the preformas coatings or in particulate form, preferably including at least aportion of the preform adjacent to the parent metal. For example, silicaadmixed with an alumina bedding is particularly useful for aluminumparent metal oxidized in air. Application of any of the dopants to thepreform may also be accomplished by applying a layer of one or moredopant materials to and within the preform, including any of itsinternal openings, interstices, passageways, intervening spaces, or thelike, that render it permeable.

The invention is further illustrated by the following example.

EXAMPLE

Aluminum Association 712.2 aluminum casting alloy ingot measuring 1 inchby 21/2 inches by 81/4 inches was placed horizontally upon a layer of amixture of commercial 8-14 grit pure alumina (Norton Co., 38 Alundum)and 5 weight percent 500-mesh SiO₂ (Pennsylvania Glass and Sand Co.) andwas subsequently covered with the same material to a depth ofapproximately three inches. The 712.2 alloy comprised, by weightpercent, about 5 to 6.5% zinc, about 0.25% or less copper, about 0.4% to0.6% chromium, about 0.15% or less silicon, about 0.40% or less iron,about 0.25% or less to 0.50% magnesium, about 0.10% or less manganese,about 0.15% to 0.25% titanium, about 0.20% or less of other metals withthe maximum amount of any one other metal being about 0.05% or less, andthe balance being aluminum.

The alumina-embedded ingot was contained within a suitable refractorycrucible and the entire assembly was placed into an air atmospherefurnace. The furnace allowed the entry of ambient air through naturalconvection and diffusion through random openings in the furnace walls.The assembly was processed for 144 hours at a setpoint temperature of1000° C. after allowing an initial eight-hour period for the furnace toreach the setpoint temperature. After the 144 hour heating period, eightadditional hours were allowed for the sample to cool to below 600° C.,after which the resulting ceramic composite was removed from thefurnace. The ceramic composite contained residual zinc, aluminum andsilicon.

In order to remove at least a substantial portion of the residual zinc,aluminum, and silicon, the ceramic composite was again contained withina refractory crucible, placed into the air furnace, and was processedfor eight hours at a setpoint temperature of 1400° C. after allowing aninitial eight-hour period for the furnace to reach the setpointtemperature. After the eight-hour heating period, eight additional hourswere allowed for the ceramic composite to cool to below 600° C., afterwhich the ceramic composite was removed from the furnace. The aluminamatrix changed from a gray, metallic color to a white color after thesecond heating step of 1400° C., indicating very little presence ofresidual metal. The microstructure of the ceramic composite revealed avery homogeneous, porous, fine-grained (approximately 6 micron diameter)alumina matrix. The residual zinc volatilized, effectively driving offany residual aluminum and silicon and providing space for in situoxidation of some of the aluminum during the second heating step at1400° C., ultimately creating a more porous, low metal content ceramiccomposite. The second heating step at 1400° C. caused no furthersubstantial oxidation reaction product growth beyond the originaldefined boundary of the composite, even though aluminum, zinc, andsilicon metals were present prior to a second heating at 1400° C. Bendtesting showed a MOR (room temperature) of approximately 4000 psi forthe final composite, and a strength retention (MOR) of about 2400 psiafter five rapid heat-up and cool-down cycles between room temperatureand 1200° C. with ten-minute soak periods at each temperature. X-rayanalysis of the ceramic product showed alumina and some minor amounts ofzinc aluminate.

To examine the effect of molten steel on this ceramic product, theceramic product was cut into four pieces and engaged to four sampleholders threaded to a bearing-supported shaft of a spin test apparatusconsisting of a steel frame holding a variable speed electric motorconnected to the bearing-supported shaft. The four pieces of ceramicproduct were rotated with the sample holders about the central axis ofthe bearing-supported shaft. The outer edge of each of the ceramicproduct pieces traveled at 600 inches per minute when rotated at 48 rpm.A sheet grade steel (low carbon, sulfur, phosphorus, and oxygen) washeated to 1593° C. and the surface deslagged prior to the start of thetest. The four pieces of ceramic product were heated to 1093° C. andthen immersed in the molten steel and rotated at 48 rpm by the spin testapparatus for 20 minutes. The four pieces of ceramic product wereremoved from the sample holders, cooled, and examined to determine theeffect of molten steel upon the ceramic product. It was determined thatthe ceramic product resisted significant penetration of steel, did notreact to any extent with the liquid steel, and did not fracture duringthe test due to any thermal gradients. Thus, the ceramic compositeproduct appears to be a useful steel refractory, such as for slide gatevalves that are in contact with molten steel.

What is claimed is:
 1. A method for producing a self-supporting ceramiccomposite comprising (1) a ceramic matrix obtained by oxidation of aparent metal comprising an aluminum alloy to form a polycrystallinematerial comprising an oxidation reaction product of the parent metalwith at least one oxidant; and (2) at least one filler embedded by thematrix, which method comprises:(a) positioning a parent metal,comprising an aluminum alloy having at least about 1% by weight zinc,adjacent to a permeable mass of filler having at least one definedsurface boundary and orienting said parent metal and said fillerrelative to each other so that formation of an oxidation reactionproduct of the parent metal with an oxidant will occur into said mass offiller and in a direction towards said defined surface boundary; (b)heating said parent metal to a first temperature above its melting pointbut below the melting point of said oxidation reaction product to form abody of molten parent metal and reacting the molten parent metal withsaid oxidant at said first temperature to form said oxidation reactionproduct, and at said first temperature maintaining at least a portion ofsaid oxidation reaction product in contact with and extending betweensaid body of molten metal and said oxidant, to draw molten metal throughthe oxidation reaction product towards the oxidant and towards and intothe adjacent mass of filler so that fresh oxidation reaction productcontinues to form within the mass of filler at an interface between theoxidant and previously formed oxidation reaction product, and continuingsaid reaction for a time sufficient to infiltrate said mass of filler tosaid defined surface boundary, with said ceramic matrix, said ceramicmatrix containing at least some residual non-oxidized metallicconstituents of said parent metal; and (c) heating the resultinginfiltrated mass of step (b) in at least one environment selected fromthe group consisting of an oxygen-containing atmosphere, an inertatmosphere and a vacuum to a second temperature above the firsttemperature but below the melting point of the oxidation reactionproduct to remove or oxidize at least a substantial portion of saidresidual non-oxidized metallic constituents of said parent metal withoutsubstantial formation of oxidation reaction product beyond said definedsurface boundary, thereby producing a self-supporting ceramic composite.2. The method of claim 1, wherein at least one dopant in addition tozinc is used in conjunction with the parent metal.
 3. The method ofclaim 1 or claim 2, wherein said filler comprises from about 3% byweight to about 10% by weight silica.
 4. The method of claim 1 or claim2, wherein said oxidant comprises an oxygen-containing gas and saidoxidation reaction product comprises an oxide of aluminum.
 5. The methodof claim 4, wherein said oxidant comprises air at atmospheric pressure.6. The method of claim 1 or claim 2, wherein said first temperature isfrom about 850° C. to about 1450° C.
 7. The method of claim 1, whereinsaid first temperature is from about 950° C. to about 1100° C.
 8. Themethod of claim 1, wherein said second temperature is greater than about1250° C.
 9. The method of claim 1, wherein said second temperature is atleast about 1400° C.
 10. The method of claim 1, wherein heating step (c)to said second temperature is effected in air at atmospheric pressure.11. The method of claim 1 or claim 2, wherein said filler comprises atleast one metal oxide, boride, nitride, or carbide of a metal selectedfrom the group consisting of aluminum, cerium, hafnium, lanthanum,silicon, neodymium, praseodymium, samarium, scandium, thorium, uranium,titanium, yttrium, and zirconium.
 12. The method of claim 1 or claim 2,wherein said filler comprises a material selected from the groupconsisting of granules, fibers, tubes, refractory fiber cloth, andmixtures thereof.
 13. The method of claim 1 or claim 2, wherein saidfiller comprises at least one of alumina and silicon carbide.
 14. Themethod of claim 1 or claim 2, wherein said ceramic matrix resulting fromsaid heating step (c) comprises interconnected porosity having at leasta portion being accessible from at least one surface of said ceramiccomposite.
 15. The method of claim 14, wherein said interconnectedporosity comprises openings having a mean diameter of less than about 6microns.
 16. A method for producing a self-supporting ceramic compositecomprising (1) a ceramic matrix obtained by oxidation of a parent metalcomprising an aluminum alloy to form a polycrystalline materialcomprising an oxidation reaction product of the parent metal with atleast one oxidant; and (2) at least one filler embedded by the matrix,which method comprises:(a) positioning a parent metal, comprising analuminum alloy having about 4-7% by weight zinc, adjacent to a permeablemass of filler having at least one defined surface boundary andorienting said parent metal and said filler relative to each other sothat formation of an oxidation reaction product of the parent metal withan oxidant will occur into said mass of filler and in a directiontowards said defined surface boundary; (b) heating said parent metal toa first temperature above its melting point but below the melting pointof said oxidation reaction product to form a body of molten parent metaland reacting the molten parent metal with said oxidant at said firsttemperature to form said oxidation reaction product, and at said firsttemperature maintaining at least a portion of said oxidation reactionproduct in contact with and extending between said body of molten metaland said oxidant, to draw molten metal through the oxidation reactionproduct towards the oxidant and towards and into the adjacent mass offiller so that fresh oxidation reaction product continues to form withinthe mass of filler at an interface between the oxidant and previouslyformed oxidation reaction product, and continuing said reaction for atime sufficient to infiltrate said mass of filler to said definedsurface boundary, with said ceramic matrix, said ceramic matrixcontaining at least some residual non-oxidized metallic constituents ofsaid parent metal; and (c) heating the resulting infiltrated mass ofstep (b) in at least one environment selected from the group consistingof an oxygen-containing atmosphere, an inert atmosphere and a vacuum toa second temperature above the first temperature but below the meltingpoint of the oxidation reaction product to remove or oxidize at least asubstantial portion of said residual non-oxidized metallic constituentsof said parent metal without substantial formation of oxidation reactionproduct beyond said defined surface boundary, thereby producing aself-supporting ceramic composite.
 17. The method of claim 16, whereinsaid first temperature is from about 850° C. to about 1450° C.
 18. Themethod of claim 16, wherein said first temperature is from about 950° C.to about 1100° C.
 19. The method of claim 16, wherein said secondtemperature is greater than about 1250° C.
 20. The method of claim 16,wherein said second temperature is at least about 1400° C.
 21. Themethod of claim 16, wherein said ceramic matrix resulting from saidheating step (c) comprises interconnected porosity having at least aportion being accessible from at least one surface of said ceramiccomposite.
 22. The method of claim 21, wherein said interconnectedporosity comprises openings having a mean diameter of less than about 6microns.
 23. The method of claim 1 or claim 2 wherein said fillercomprises at least one whisker.